The following is related generally to the optical components used in optical communication networks, and specifically to optical devices that can amplify optical signals.
Erbium-Doped Fiber Amplifiers (EDFAs) or Praseodymium-Doped Fiber Amplifiers (PDFAs) are widely deployed in optical networks, in the 1550 nm or 1310 nm wavelength windows, respectively. FIG. 1 illustrates the multiple optical components that are commonly included in an EDFA or PDFA. The optical power from the pump laser light source 102 is combined with the input signal 101, by a wavelength-division multiplexing (WDM) coupler 104. The combined input signal and pump laser light then passes through a section of fiber 103 that has been doped with Erbium or Praseodymium ions in its core. The pump laser light excites the Erbium or Praseodymium ions embedded in the erbium-doped (or praseodymium-doped) fiber 103 to a higher energy level. The optical input signal 101 then induces stimulated emission and is therefore amplified to create the output signal. However, amplified spontaneous emission (ASE) noise is also generated simultaneously, and creates noise on top of the amplified input signal 101. Thus, the output signal 106 consists of an amplified input signal, as well as the ASE noise component. An isolator 105 is located after the erbium-doped or praseodymium-doped fiber 103. This isolator 105 is intended to prevent the back-scattering power out of the downstream optical fiber and other components from re-entering the EDFA or PDFA. This unwanted back scattering power would otherwise be amplified, and would therefore interfere with the EDFA's (or PDFA's) normal characteristics and performance. Also shown in FIG. 1 is a pump laser monitoring port 107.
In order to reduce the size and cost of an optical amplifier, it is desirable to replace the loop of doped fiber (item 103 in FIG. 1) with an optical waveguide structure. FIGS. 2A, 2B, and 2C show several types of optical waveguides that are commonly used for integrated light wave circuits. An optical waveguide consists of a core with higher refractive index than the cladding portions of the waveguide structure, in order to confine and guide light along the waveguide through total internal reflection. FIG. 2A shows a planar waveguide consisting of a core layer 201 surrounded by two cladding layers, a cover layer 202 and a substrate 203, which have lower refractive index than the core 201. The power of the optical wave is confined in the core layer in the z-direction and can propagate freely in the x-y plane. The thickness of the core layer can range from a few hundred nanometers to tens of micrometers, depending on the wavelength and desired number of optical modes. The electric field intensity of the fundamental guided mode 204 has a peak power at around the center of the core and its intensity is reduced at the two interfaces to the cladding layers. A portion of the electric field can penetrate into the cladding layers, which have lower refractive indices, this is known as the evanescent field of the guided mode. The penetration depth of the evanescent field into the claddings depends on the refractive index contrast between the core and the claddings, as well as the core thickness. Lower index contrast and a smaller core will result in larger penetration depth.
A buried channel waveguide can be used to confine light in two directions, and as shown in FIG. 2B, the light is confined in both the z-direction and the x-direction, and propagates in the y-direction. The core 207 is surrounded by the cover layer 205 and the substrate 206, which have lower refractive indices. Most of the power of the guided mode is confined inside the core 207, although some of the evanescent field 208 extends into the claddings 205 and 206. The propagation constants of the two polarization states, referred to as the transverse electric (TE) and transverse magnetic (TM) modes, are dependent on the core geometry and refractive index contrast. For optical amplifier applications, it is often desirable to have the same or similar propagation constant for both polarizations to reduce the dispersion effect and polarization dependent gain/loss.
As shown in FIG. 2C, confinement in the x-direction can also be achieved by fabricating a rib structure 209 on top of, or adjacent to, the core layer 213. This is called a rib waveguide and is usually fabricated by etching away some of the core material to form a protruding rib. The height and width of the rib determines the power confinement of the guided mode. A shallow rib will result in weak confinement and the electric field intensity of the guided mode 210 will be extended in the x-direction. The rib structure 209 can also be fabricated from materials different than the core layer 213, with appropriately chosen refractive index.
FIG. 3A shows one example of a prior art embodiment of a waveguide-based optical amplifier, where the pump laser 301 and optical signal 302 are injected into the same core 305, surrounded by claddings 304. The waveguide structure would usually be implemented using a buried channel waveguide, or a rib waveguide, as shown in FIGS. 2B and 2C, respectively. The core 305 can be doped with rare-earth elements such as Erbium ions for signal amplification at around 1550 nm, or Praseodymium ions for signal amplification at around 1310 nm, for fiber optics communication applications. (In the discussion that follows, the use of Erbium ions will be assumed. One skilled in the art will recognize that other rare-earth ions can also be used, for signal wavelengths that are not in the vicinity of 1550 nm.) As the pump light propagates along the waveguide, it is being absorbed by the Erbium ions, which are excited to higher energy level(s). An excited Erbium ion can relax to the ground state through emission of a photon with wavelength longer than the pump light source, either through stimulated or spontaneous emission. The optical signal in the waveguide can stimulate the emission of a photon from an excited Erbium ion, with the same wavelength and properties (e.g. polarization, coherence) as the signal photons. Such stimulated emission is used to transfer energy from pump to signal, through the excitation and relaxation of the Erbium ions. As the optical signal propagates along the waveguide, it gains energy from the Erbium ions and its power increases. At the end of the waveguide, the signal 303 has been amplified, with gain on the order of a few dB, to tens of dB, while the pump intensity is greatly attenuated. However, the spontaneous emission can have different wavelength, polarization and coherence properties, compared to the optical signal. Therefore, the spontaneous emission represents an undesirable artifact of the optical amplifier. The spontaneous emission will be guided by the waveguide core 305, and will be amplified as well (306). This is called amplified spontaneous emission (ASE), and it is a dominant noise source that reduces the signal-to-noise ratio (SNR) of an optical amplifier. Furthermore, ASE also reduces the total amplifier gain since part of the pump energy is used to amplify the ASE noise instead of the input optical signal.
Instead of injecting the pump light at the same input port as the signal, it can also be injected from the output port, in which case the pump is propagating in the opposite direction as the signal. Bi-directional pumping has been adopted as well, to provide more uniform gain along the waveguide. FIG. 3B illustrates another waveguide amplifier which integrates laser diodes 353 as pump sources, coupled into a waveguide 351 which has been doped with rare-earth ions. The pump laser light from multiple laser diodes is injected into the waveguide 351 through evanescent coupling, instead of the physical “Y”-split junction that was shown in FIG. 3A.